The present invention relates to a navigation system for medical devices based on the use of magnetic fields. More particularly, this invention relates to a method and system for determining the position and orientation of a catheter probe being used during a surgical procedure in the presence of extraneous objects that may introduce extraneous magnetic fields.
Systems and methods for determining the position and orientation of surgical probes based on the use of magnetic fields are known. See, for example, U.S. Pat. No. 5,592,939, herein incorporated by reference. Such systems and methods generally rely on the ability to solve a known equation for a known field strength in order to obtain the unknown position and orientation variables. Although the focus here is a rigid catheter probe of known length, width and depth, one skilled in the art will appreciate that the techniques discussed here are equally applicable to other types of probes; for example, the techniques discussed here may be adapted to the use of a flexible probe.
In general, if the position in three-dimensional space of two points within the rigid probe is known, then the position and orientation of the probe itself is known. Each unknown point P in space corresponds to three unknown variables as shown in FIG. 1. These variables can be what are commonly called x, y, and z in a Cartesian system as is shown in FIG. 1, or they can be the variables r,θ, and φ as are commonly used in spherical coordinates also as shown in FIG. 1. Two unknown points in space correspond to 6 unknown variables. However, when the two points are on a rigid catheter, and when the separation of the two points is known, one unknown variable is removed. Thus, the position and orientation of a rigid catheter probe in three-dimensions is a function of 5 unknown variables. These variables can be expressed in a form that utilizes both Cartesian and spherical coordinates. For example, the position of sensor coil 14 can be defined by three Cartesian coordinates x, y, and z as is shown in FIG. 2, and the orientation of sensor coil 14 along coil axis 21 is defined by the variables θ and φ, also as shown in FIG. 2.
In order to solve for 5 unknown quantities, one typically requires 5 known linearly independent equations. One can obtain known equations that are linearly independent of each other by exposing a detector located on the catheter in an unknown position and orientation to known independent navigation fields. Thus, to obtain 5 known linearly independent equations requires a sampling of at least 5 known independent navigation fields. Nevertheless, current systems that utilize magnetic fields in order to determine position and orientation frequently sample more than 5 independent fields. See, for example, U.S. Pat. No. 5,592,939, herein incorporated by reference. One of the reasons for sampling more than 5 independent fields is to perform a self-consistency check. Ideally, every sampling above 5 should provide such a system with redundant information regarding position and orientation. However, in an operating room in practice, every sampling of a known navigation field beyond 5 yields slightly different results as to the position or orientation of the catheter probe. One of the reasons for this is the nearby presence extraneous objects that are conductive or ferromagnetic. Such objects respond to the known navigation field and introduce errors that can be large depending on the nature and relative position of the object.
For example, a conducting object within the area of influence of the known navigation field can develop what is known as an eddy current in response to the known navigation field. These eddy currents can, in turn, generate an extraneous magnetic field of unknown strength and orientation in the vicinity of the catheter. Depending upon the size of the object, this effect can be large.
In addition, an object with a ferromagnetic core will act to focus magnetic field flux lines through the core and thus distort the known navigation field, again, in an unknown manner. Often, objects with ferromagnetic and conductive cores are used in surgical settings, such as tools used to drill, ream and tap holes in the vertebrae of a patient.
In light of the foregoing, it is desirable to account for the effects of conducting objects that introduce eddy currents in response to a known navigation field.
It is further desirable-to account for the effects of objects with ferromagnetic and conductive cores that introduce fluctuations in a known navigation field and that are often moved about near the periphery of the navigation field, such as surgical tools.
It is further desirable to account for the effects of objects that introduce arbitrary fluctuations in a known navigation field.